1. Field of the Invention
The invention relates to a control device and a control method for a vehicle power transmission device. In particular, the invention relates to an engine stop control during a neutral state in a vehicle power transmission device that includes an electrical differential portion in which the differential state of a differential mechanism is electrically controlled and an engagement device that constitutes a portion of a power transmission path.
2. Description of the Related Art
There is a known vehicle power transmission device that includes an electrical differential portion in which the differential state between the rotation speed of an input shaft and the rotation speed of an output shaft is controlled through the control of the state of operation of a first electric motor linked to a rotating element of a differential mechanism, and an engagement device that constitutes a portion of a power transmission path between the electrical differential portion and driving wheels. An example of this device is a vehicle power transmission device described in Japanese Patent Application Publication No. 2006-46541 (JP-A-2006-46541). In this hybrid type vehicle power transmission device, the differential mechanism is constructed of, for example, a planetary gear device, and the first electric motor is linked to a rotating element of the differential mechanism so as to be capable of power transmission. In the vehicle power transmission device of JP-A-2006-46541, when the engine is to be stopped, a lock mechanism (a switching clutch C0 and a switching brake B0) that is provided in the electrical differential portion and that restricts the differential state is put into a released state, and the engine rotation speed is promptly reduced by using the first electric motor, so that the engine rotation speed promptly passes through an engine rotation speed region in which resonance of a power transmission system occurs.
In the vehicle power transmission device of JP-A-2006-46541, however, if the engine stop control is executed while the ratio shift portion is in the neutral state (power transmission disconnected state), the inertia of a rotating element of the differential mechanism linked to the ratio shift portion (which corresponds to a ring gear in JP-A-2006-46541) is lessened since the rotating element is not linked to the driving wheels. If during this state, torque (negative torque) in a negative direction (reverse rotation direction) acts on the first electric motor, the momentum of the negative torque causes the rotation speed of the rotating element (ring gear) linked to the ratio shift portion to become high due to the differential motion effect. Thus, there is possibility of decline of the durability of various components and the like, including the seal component parts and the bearings that support the differential mechanism.